Study on the Fast Nitriding Process of Active Screen Plasma Nitriding

Study on the Fast Nitriding Process of Active Screen Plasma Nitriding

Available online at www.sciencedirect.com ScienceDirect Physics Procedia 50 (2013) 94 – 102 International Federation for Heat Treatment and Surface ...

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Available online at www.sciencedirect.com

ScienceDirect Physics Procedia 50 (2013) 94 – 102

International Federation for Heat Treatment and Surface Engineering 20th Congress Beijing, China, 23-25 October 2012

Study on the fast nitriding process of active screen plasma nitriding L.Han*a, b, , J.T.Daia, X.R. Huangb, C.Zhaob 

a

a Aviation Mechanical Engineering Department, Qingdao Branch, Navy Aeronautical Engineering Academy, Qingdao 266041, China Surface Engineering Laboratory, College of Electromechanical Engineering, Qingdao University of Science & Technology, Qingdao 266061, China

Abstract A new nitriding technology using active screen plasma nitriding (ASPN) was carried out in order to increase the nitriding speed of AISI 5140 steel. The fast nitriding process is based upon the different solubility and diffuse rate of nitrogen atoms in austenite and in ferrite of steel respectively. First, the nitriding samples were heated above the eutectoid temperature for a few minutes to dissolve a large amount of nitrogen and form a nitrogen-rich layer on the surface of the nitrided samples. Then, the nitriding temperature was decreased below the eutectoid temperature and kept the temperature for a long time to make the dissolved nitrogen in the nitrogen-rich layer diffuse into ferrite. The two different nitriding processes were carried on alternately. Experimental results indicate that the fast nitriding process not only enhances the nitriding speed remarkably, but also keeps the high hardness of the nitrided layer. The new fast nitriding technology with nitrogen-rich layer can be explained with “absorptiondiffusion” model. © 2013 © 2013 The The Authors. Authors.Published Publishedby byElsevier ElsevierB.V. B.V. Selection and Society. Selection and peer-review peer-reviewunder underresponsibility responsibilityofofthe theChinese ChineseHeat HeatTreatment Treatment Society Keywords: active screen plasma nitriding (ASPN); fast nitriding; nitrogen-rich layer; “absorption-diffusion” model

1. Introduction Nitriding may enhance the surface hardness and wear-resistance of steels remarkably. The distortion of the nitrided work pieces is also quite small because the nitriding is carried out below the phase-transform temperature. However, the nitriding time is very long owing to the low diffuse rate of nitrogen atoms in steel at the low nitriding temperature. This has become the biggest obstacle in the application of nitriding technology. Many efforts have been made to shorten the nitriding time, such as two-step nitriding[1], rare-earth accelerated nitriding[2], plasma nitriding and so on. * HAN Li. Tel.: 86-13863924056. E-mail address: [email protected]

1875-3892 © 2013 The Authors. Published by Elsevier B.V. Selection and peer-review under responsibility of the Chinese Heat Treatment Society doi:10.1016/j.phpro.2013.11.017

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Active screen plasma nitriding (ASPN) is a new type of plasma nitriding technology[34]. The unique structure of ASPN apparatus makes it convenient to investigate the nitriding process and nitriding mechanism[5]. In this paper, a new fast nitriding technology will be studied by means of the technology of ASPN. 2. Experimental detail 2.1 Experimental apparatus Fast plasma nitriding experiments were carried out using a self-made ASPN unit[6] as shown in Fig.1, which comprised a sealed chamber, vacuum system, DC power supply, feed gas system, and a temperature control system. A mesh steel cage of 120 mm diameter and 100 mm height was mounted in the vacuum chamber and connected to the cathode of the DC power supply. The furnace wall was connected with the anode and earthed. The samples are located in a floating potential and isolated from the cathodic mesh screen and the anodic furnace wall with ceramic and mica pieces. Temperature was measured with a thermocouple inserted into a hole of 3mm diameter in a dummy sample which had the same dimensions of the samples and was placed in the symmetrical positions as the samples.

Fig. 1 Schematic diagram of the ASPN experimental set-up

2.2 Material The material used in this experiment was AISI5140 medium carbon steel with chemical compositions given in Table 1. The samples are disc of 18 mm in diameter and 4 mm in thickness. The flat surfaces of the disc were manually ground using silicon carbide grinding papers down to 1200 grade to achieve a fine finish. The samples were cleaned in acetone and ethanol respectively before the nitriding treatment. Original metallograph of AISI5140 steel by quenching and tempering treatment is shown in Fig.2. Table 1 Chemical compositions of AISI 5140 steel

C 0.37~0.45

Mn 0.50~0.80

chemical compositionwt.% Si Cr 0.20~0.40 0.80~1.10

Fe Bal.

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Fig.2 Original metallograph of AISI 5140 steel by quenching and tempering

2.3 Processing The treatments were carried out using the parameters as shown in Table 2. After the chamber was evacuated to 5 Pa, hydrogen and nitrogen were introduced into the chamber and the DC power supply was switched on. The process timer started at once when the desired nitriding temperature was reached. The experiments adopted three different nitriding processes, nitriding at low temperature which is 550 (ASPNL), nitriding at high temperature which is 600  (ASPNH) and fast nitriding process (ASPNF) respectively. The process curves of ASPNL, ASPNH and ASPNF are shown in Fig.3. In the fast nitriding process, the sample was heated up to 600 and maintained the temperature for 10 minutes (tH). Then, the sample temperature was lowered to 550 and maintained the temperature for 50 minutes (tL). The two different nitriding steps were carried out alternately for eight times within eight hours. After the nitriding treatment, the DC power supply was switched off and the samples were cooled down to the room temperature in the furnace. Table 2 Process parameters for ASPN

No. 1 2 3

Nitriding models Low temperature ASPNL High temperature ASPNH Fast nitriding ASPNF

Current /A

Voltage /V

N2: H2

Pressure /Pa

Time /h

Temperature /

1.3

620

2575

300

8

550

1.5

720

2575

360

8

600

2575

330~360

8

550~600

1.3~1.5

720~620

L. Han et al. / Physics Procedia 50 (2013) 94 – 102

(a)

(b) Fig.3 Process curves of ASPN (a) ASPNL and ASPNH, (b) ASPNF

2.4 Characterization of ASPN samples Transverse sections of the nitrided samples were polished and chemically etched in 3% nital solution. The nitrided microstructure was examined with an optical microscope (OM). The phase structures were analyzed by D/max-A X-ray diffraction (XRD) using Cu-Kradiation (v= 10°/min). A Vickers micro hardness tester was applied to measure the hardness of the surface and cross-sectional of the nitrided samples. 3. Experimental results 3.1 OM observation Fig.4 shows the optical microstructure of the nitrided samples. It can be seen that there is a difference in metallographs between the samples nitrided by ASPNF and ASPNL. In the nitrided layer of ASPNF sample, besides a thin compound layer on the surface and a nitrogen diffusion layer, there is a dark layer under the compound layer which is named nitrogen-rich layer (Fig.4a and Fig.4b). There is no nitrogen-rich layer in the sample by ASPNL (Fig.4c). The nitrogen-rich layer is located at the sample’s surface if the sample is nitrided by ASPNH, (Fig.4d).

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Fig.4 Metallographs of AISI 5140 steel by ASPN (a) ASPNF process (high multiple), (b) ASPNF process (low multiple), (c) ASPNL process, (d) ASPNH process

3.2 Hardness and Thickness Fig.5 shows the hardness-depth profiles of the nitrided samples. Compared with the ASPNL process, the ASPNF process can produce a thicker hardened layer within the same nitriding time. The hardened layer of the sample nitrided by ASPNF is 20~30% thicker than that of the sample nitrided by the ASPNL. The hardness is the lowest when the sample is nitrided by the ASPNH process. Fig.5 also shows that there is a low hardness area of tens of micrometers under the surface of the ASPNF sample. The low hardness area is just corresponding to the nitrogen-rich layer of the ASPNF sample as shown in Fig.4a.

Fig.5 Hardness profiles of nitrided layers in different ASPN processes

3.3 X-ray diffraction analysis Fig.6 shows the XRD patterns of the nitrided layer by the ASPNL, ASPNH and ASPNF processes respectively. It can be seen that the nitriding layer nitrided by ASPNL process comprises Fe4N and Fe3N (Fig.6a). Fe2N, Fe3N and Fe4N appear in the nitriding layer nitrided by ASPNH process (Fig. 6b). Only Fe4N is found in the nitriding layer nitrided by ASPNF process (Fig. 6c).

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(a)

(b)

(c) Fig.6 XRD patterns of nitrided layer in different ASPN processes (a) ASPNL process (b) ASPNH process (c) ASPNF process

4. Discussion The nitriding process of steel includes several basic steps, such as gas phase diffusion, adsorption, chemical reaction on the gas-solid interface, absorption, and diffusion in the solid phase. In the initial steps of the nitriding treatment, adsorption is important factors to control the nitriding speed. As soon as the nitrogen atoms on the interface is close to saturation, the solid phase diffusion of nitrogen atoms in the steel should be regarded as the main factor to control the nitriding speed. The effective method increasing nitriding speed is to find out the controlling factors at different steps of each circulation, so as to make adsorption and inside-diffusion completely match. During nitriding process, the important steps are adsorption and solid phase diffusion. According to the Fick’s Second Law

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C  2C D 2 t x

(1)

Where C is the volume concentration of a diffusion element (kg/m3), x is the diffusion distance (m), t is the diffusion time (s), and D is the diffusion coefficient (m2/s). x is the change rate of diffusion element concentration along the x direction namely the concentration gradient. In the diffusion step the concentration gradient in the nitrided layer will vary with the distance from surface as well as the nitriding time. With the diffusion coefficient of nitrogen in material getting bigger, more nitrogen is diffused and the diffused layer becomes deeper. During the nitriding process of steel, the diffusion speed of nitrogen atoms in the nitrided layer mainly depends on the nitrogen concentration on the steel surface, the concentration gradient and diffusion coefficient of nitrogen in the nitrided layer. Among the three ones, the diffusion coefficient of nitrogen in the nitrided layer plays the most important role. According to the relationship between the diffusion coefficient and temperature:

Q D D0exp( RT ) 

(2)

Where D is the diffusion coefficient (m2/s), T is the temperature (K) and D0, Q, R are constant. Therefore, the higher the temperature is, the greater the diffusion coefficient. However, this law is only suitable for the condition that the crystal structure is not changed. According to Table 3, once the nitriding temperature is higher than the eutectoid temperature590, the diffusion speed of nitrogen atoms in austenitic is very low. The diffusion coefficient of nitrogen atoms in austenitic Q is 2000 times less than that of nitrogen atoms in ferrite R. Therefore, it is not feasible an effective method to increase the diffusion speed only by raising nitriding temperature. Table 3 Relationship between temperature and diffusion coefficient of nitrogen in R and Q[7]

diffusion temperature 

D of nitrogen in ×10-11 cm2s-1

D of nitrogen in

×10-11 cm2s-1

550 600 700

760 1428 /

/ 0.70 5.48

Another characteristic of nitrogen in Q-phrase is high solubility of nitrogen. The solubility of nitrogen in Qphrase is much higher than that of nitrogen in R-phrase because there is a large space in the center of the FCC structure of Q-phrase, which is much bigger than that of BCC structure of R-phrase. According to Fig.7, the solubility of nitrogen in Q-phrase is about 2.35% at the eutectoid temperature of 590, but the solubility of nitrogen in R-phrase is only 0.11% at the same temperature. The nitrogen solubility of the former is above 20 times more than that of the latter.

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Fig.7 Phase diagram of Fe-N [7]

Based on the facts that the nitrogen solubility in Q phase is higher and the diffusion speed of nitrogen in R phase is higher, a new idea of fast nitriding process (ASPNF) is advanced. First, the substrate is heated above the eutectoid temperature (for example 600) and keep the temperature for a short time to make the substrate’s surface form a thin nitrogen-rich layer in order to increase the concentration and concentration gradient of nitrogen atoms. Second, the nitriding temperature is decreased below the eutectoid temperature (for example 550) and keep the temperature for a long time to make the nitrogen atoms in the nitrogen-rich layer quickly diffuse into the substrate. Because this diffusion is carried out on the condition of higher concentration gradient, the nitriding speed is raised. As soon as nitrogen concentration in the nitrogen-rich layer drops, the nitriding temperature is raising above the eutectoid temperature to raise the nitrogen concentration of the nitrogen-rich layer once again. The processes of nitrogen solution at a high temperature and nitrogen diffusion at low temperature are repeated several times until the thickness of the nitrided layer is formed. It should be pointed out that the nitrogen-rich layer can’t be too thick because the diffusion coefficient of nitrogen in austenitic is very small. Nitrogen atoms in the nitrogen-rich layer will be firmly fixed by the octahedron of FCC austenitic and these nitrogen atoms can not diffuse into substrate to remain in the nitrogen-rich layer. If so, the surface of the nitrided layer can not absorb new nitrogen, which will lead to the failure of the fast nitriding process. Conversely, if the nitrogen-rich layer is too thin, the nitriding speed may not be a optimum speed. Therefore, the key of the ASPNF process is how to control the thickness of the nitrogen-rich layer. In other words, the key of the ASPNF process is how to select the ASPNF process parameters, such as the temperature of high temperature nitriding, the time of high temperature nitriding, and the time of low temperature nitriding. Based on the above discussion, if the traditional nitriding process can be explained by the nitriding model of “adsorption-diffusion” (Fig.8a), then the fast nitriding process can be regarded as the model of “absorptiondiffusion” (Fig.8b).

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(a)

(b)

Fig.8 Models of different nitriding mechanisms (a) adsorption-diffusion model, (b) absorption-diffusion model

5. Conclusions (1) A new fast nitriding process is carried out by active screen plasma nitriding (ASPN), which can not only enhance the nitriding speed through controlling the process parameters but also guarantee high hardness of the nitrided layer. (2) According to the “absorption-diffusion” model, the nitriding sample absorbs a large amount of nitrogen to form nitrogen-rich layer and then the nitrogen diffuse into substrate relying on high concentration and high concentration gradient. The nitriding speed is increased by repetition of “absorption-diffusion” process. Nitrogen-rich layer which is too thin or too thick will affect nitriding effect. Therefore, nitriding low temperature, nitriding high temperature and the corresponding time are the important parameters of fast nitriding ASPN process. (3) The nitriding speed of ASPNF process is 20~30% faster than that of ASPNL process.

Reference [1] Shen Site, Zhu Peidi. Transactions of Metal Heat Treatment, 1986, 7 (2):11-14 (in Chinese) [2] Peng J, Dong H. Effect of Rare Earth Elements on Plasma Nitriding of 38CrMoAI Steel. Surface Engineering1996, 12(2):151. [3] J Georges, TC plasma nitriding, US patent 5,989,363, No.23. 1999 [4] C.X.Li, T.Bell: Principles, Mechanisms and Applications of Active Screen Plasma Nitriding, Heat Treatment of Metals, 2003, (1):1-7 [5] C.Zhao, C.X.Li, H. Dong and T.Bell: Study on the active screen plasma nitriding and its nitriding mechanism, Surface Coatings Technology, 2006, 201, (6):2320-2325 [6]C.Zhao. Adiabatic Plasma Apparatus of Metal Surface Treatment. China Patent, No.012300209. 2001 [7] Beijing Research Institute of Mechanical & Electrical Technology, Nitriding microstructure map of Ion & steel materials [M], Beijing: Machinery Industry Press, 1986 (in Chinese)